The present disclosure, in various exemplary embodiments, relates to electrophotographic imaging members and, more specifically, to layered photoreceptor structures with improved overcoat layers and processes for making the imaging members. Specifically, the exemplary embodiment relates to a solvent system for coating certain overcoating materials to produce a high quality surface finish.
Electrophotographic imaging members, i.e. photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the dark so that electric charges can be retained on its surface. Upon exposure to light, the charge is dissipated.
An electrostatic latent image is formed on the photoreceptor by first uniformly depositing an electric charge over the surface of the photoconductive layer by one of the many known means in the art. The photoconductive layer functions as a charge storage capacitor with charge on its free surface and an equal charge of opposite polarity on the conductive substrate. A light image is then projected onto the photoconductive layer. The portions of the layer that are not exposed to light retain their surface charge. After development of the latent image with toner particles to form a toner image, the toner image is usually transferred to a receiving substrate, such as paper.
Imaging members can have a number of different configurations. For example, they can comprise a flexible member, such as a flexible scroll or a belt containing a flexible substrate support. The flexible member belt may be seamed or unseamed. The electrostatographic imaging members can also be a rigid member, such as those utilizing a rigid support substrate drum. Drum imaging members have a rigid cylindrical supporting substrate bearing one or more imaging layers. The use of such small diameter drums or flexible imaging belts places a premium on photoreceptor life. Accordingly, a need exists for improving photoreceptor life.
One approach to achieving longer photoreceptor life is to form a protective overcoat on the imaging surface, e.g. the charge transporting layer of a photoreceptor. This overcoat layer must satisfy many requirements, including transporting holes, resisting image deletion, resisting wear and avoidance of perturbation of underlying layers during coating. Although various hole transporting small molecules can be used in overcoating layers, one of the toughest known overcoatings includes cross-linked polyamide (e.g. LUCKAMIDE) containing N,N′-diphenyl-N,N′-bis(3-hydroxyphenyl)-[1,1′-biphenyl]-4,4′-diamine (DHTBD). This overcoat is described in U.S. Pat. No. 5,368,967, the entire disclosure thereof being incorporated herein by reference.
Known overcoat solutions used on drums or flexible imaging belts are generally not easily coated on web photoreceptors at speeds over one meter per minute (m/min). If higher coating speeds are used, the resulting coatings exhibit a dull and opaque surface quality, even for very thin layers. This problem is exacerbated in a standard alcohol solvent system by dilution to lower percent solids. Accordingly, a need exists for a strategy for coating overcoating materials, and which is particularly adapted for use in high speed coating operations.